Considering first the prior art relating to air circulating fireplaces, it is known to construct fireplaces or inserts therefor which provide means to circulate room air through passages in the walls defining the combustion chamber to absorb heat from the source, after which the heated air is returned to the room. This art includes elaborate labyrinthian passages for the room air and combustion air alike in an attempt to lengthen the period of residence of the respective flows in mutual heat exchange relationship, as exemplified by U.S. Pat. No. 2,642,859, issued June 23, 1953 to Newman T. Brown. Moreover, it has been proposed to so dimension the combustion chamber that an unconfined slowly descending recirculating flow of combustion air is encouraged, as seen in U.S. Pat. No. 773,863, issued Nov. 1, 1804 to Mary F. Frecktling, and to provide confined passages to direct a recirculating flow, as in U.S. Pat. No. 2,821,975, issued Feb. 4, 1958 to Robert K. Thulman, in U.S. Pat. No. 2,185,788, issued Jan. 2, 1940 to August R. Fredlund, and in U.S. Pat. No. 53,880, issued Apr. 10, 1866 to Francis M. Rogers. It is noted that the Frecktling disclosure recirculates only the slowly moving portion of the combustion products, the principle heat containing portion passing directly to the flue. On the other hand, the other disclosures in which substantial portions of the combustion flow are recirculated in confined paths requires the introduction of structural impedance to the gas flow and depends upon the presence of a large expanse of heat exchange surface.
Turning to the art relating to combustion generally, it is well known to induce a helical flow of a fuel/air mixture in order to increase the residence time of the mixture within the combustion zone and thus enhance complete combustion, and it has been suggested that such an effort may be augmented by restricting the outlet of the combustion chamber or by introducing a supplemental forced air flow. For an example of this art, reference is to U.S. Pat. No. 3,007,310, issued Nov. 7, 1961 to Karl Eisele and to U.S. Pat. No. 3,258,052 issued June 28, 1966 to Alfred Wilson, et al. Augmentation of the spiral flow of air/fuel mixture has also been proposed by flow conditions which induce an annular tore comprising a flame vortex adjacent the base of the flame in U.S. Pat. No. 3,030,773, issued Apr. 24, 1962 to Robert H. Johnson and in U.S. Pat. No. 3,255,802 issued June 14, 1966 to James A. Browning. A similar flow is induced within the area of air/fuel mixing in U.S. Pat. No. 3,118,489, issued Jan. 21, 1964 to Clifford C. Anthes.
Considering the prior art even more generally, in the field of heat exchange it is known to induce a gaseous medium to flow in a vortical pattern extending axially of a tubular conduit in order to increase residence time, enhance scrubbing action and to obtain an interchange of position of the molecules of high velocity and temperature gases from the center of the vortex with the outer molecules which have been deprived of their energy and velocity through functional heat exchange contact with the vortex tube in which the vortical flow is confined. This is exemplified by the well known "Ranque" tube (U.S. Pat. No. 1,952,281, Mar. 27, 1934) and see U.S. Pat. No. 2,586,002, issued Feb. 19, 1952 to W. R. Carson, Jr. et al.
In summary, the prior art is known to disclose inducement of vortical flow in precombustion gases and basal portions of flame patterns for the purpose of enhancing the mixing of the fuel air mixture, and the prior art discloses inducement of hot gas vorticity axially of confined conduits of heat exchangers.